This invention relates to semiconductor integrated circuits constructed by alternately connecting combinational logic circuits and flip-flops.
In a semiconductor integrated circuit having a construction in which flip-flops are alternately connected with combinational logic circuits, data processed in a combinational logic circuit in the preceding stage is captured in a flip-flop synchronously with a clock signal, and the processed data is transferred to a combinational logic circuit in the succeeding stage.
Semiconductor integrated circuits may be designed using low power-consumption design techniques such as MTCMOS (Multi-Threshold CMOS) technique. The low power-consumption design techniques employ combinational logic circuits and flip-flops that are able to be switched between an active mode and a sleep mode. Supply of electric power to unnecessary portions of the integrated circuit is terminated when in the sleep mode. For example, see U.S. Pat. No. 6,404,254, which is incorporated by reference in its entirety.
In the low power-consumption design, retention flip-flops may be used in addition to standard or non-retention flip-flops. During the sleep mode, supply of electric power to the non-retention flip-flops is terminated. Meanwhile, during sleep mode, the supply of electric power is maintained to a portion of the retention flip-flops necessary to retain the data. For example, see U.S. Pat. Nos. 6,437,623, 6,870,412, 6,965,261, 6,989,702, 7,154,317, 7,164,301, 7,170,327, 7,183,825, 7,227,383, and 7,332,949, which are incorporated by reference in their entireties.
In the circuitry including retention flip-flops and non-retention flip-flops, it is required to initialize the non-retention flip-flops without initializing the retention flip-flops when returning from the sleep mode to the active mode. That is, the non-retention flip-flops, which do not retain the data during the sleep mode, should be initialized so that they do not output irregular data when returned to the active mode. The retention flip-flops, which retain the data during the sleep mode, should not be initialized so that they output the data retained therein when returned to the active mode.
The non-retention flip-flops may be initialized by, for example, supplying initialization signal to the non-retention flip-flops through a signal line specially provided for supplying the initialization signal. However, providing the special line requires an additional task during the design of the semiconductor integrated circuit. Furthermore, it is necessary to provide a circuit that detects the switching from the sleep mode to the active mode and generates the initialization signal.